1. Field of the Invention
The present invention relates to comparators. In particular, the present invention relates to chopper comparators. Further, the present invention relates to semiconductor devices including chopper comparators. Furthermore, the present invention relates to electronic devices including the semiconductor devices.
2. Description of the Related Art
A differential amplifier (also referred to as an operational amplifier) and a chopper comparator (also referred to as a chopper inverter comparator, an inverter chopper comparator, or the like) are widely used as comparators (comparison circuits). In Patent Document 1 (see FIG. 1 of Patent Document 1), a chopper comparator as illustrated in FIG. 9 is used.
In FIG. 9, a chopper comparator includes an inverter 2621, a capacitor 2622, and switches 2624, 2625, and 2626. The switch 2624 is electrically connected in parallel to the inverter 2621. An output terminal of the inverter 2621 is electrically connected to an output terminal (indicated by “OUT” in FIG. 9) of the chopper comparator. An input terminal of the inverter 2621 is electrically connected to one of a pair of electrodes of the capacitor 2622. Here, the input terminal of the inverter 2621 or the one of the pair of electrodes of the capacitor 2622 is referred to as a node M (indicated by “M” in FIG. 9). The other of the pair of electrodes of the capacitor 2622 is electrically connected to an input terminal (indicated by “IN” in FIG. 9) of the chopper comparator through the switch 2626, and is electrically connected to a terminal VR to which a reference potential is applied, through the switch 2625.
In the chopper comparator in FIG. 9, a reference potential Vref is input to the terminal VR when the switch 2626 is turned off and the switches 2625 and 2624 are turned on. Thus, the potential of the node M is set to the threshold voltage (hereinafter also referred to as Vthi) of the inverter 2621. Here, the threshold voltage of the inverter 2621 corresponds to an input potential (or an output potential) under the condition that the input potential and the output potential of the inverter 2621 are equal. To set the potential of the node M to Vthi is also referred to as initialization. Note that initialization operation can also be referred to as operation of holding predetermined voltage (hereinafter also referred to as reference voltage or criterion voltage Vc) between the pair of electrodes of the capacitor 2622. Here, the reference voltage Vc is voltage obtained by subtraction of the threshold voltage Vthi from the reference potential Vref.
After the initialization, a signal potential (hereinafter also referred to as Vin) is input to the input terminal IN when the switch 2626 is turned on and the switches 2625 and 2624 are turned off. When the signal potential Vin is higher than Vthi (a high-level potential (hereinafter referred to as VinH)), the output potential of the inverter 2621 becomes low, so that a low-level potential (hereinafter referred to as VoutL) is output from the output terminal OUT. When the signal potential Vin is lower than Vthi (a low-level potential (hereinafter referred to as VinL)), the output potential of the inverter 2621 becomes high, so that a high-level potential (hereinafter referred to as VoutH) is output from the output terminal OUT. Operation of outputting a signal corresponding to a signal input from the input terminal IN in this manner is referred to as normal operation.
The chopper comparator operates as described above.